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Green algae- Charophyceans (closest relatives to plants)
Evidence supporting relationship
Similar Characteristics
• Protists (algae) are similar to plants by : multicellular, eukaryotic, photosynthetic
autotrophs
• Plants have : cellulose in their walls and so do green algae, dinoflagellates, and brown
algae. And chloroplats with chlorophylls a and b
4 Key traints that land plants share only with charophyceans
1. Rose shaped complexes for cellulose synthesis
- The cells of both l.plants and Charophyceans have rosette cellulose synthesizing
complexes. These are rose shaped arrays of proteins in the plasma membrane that
synthesize the cellulose of the cell walls. (noncharophycean algae have linear
arrays)
- Cell walls of plants and charophyceans have higher percentage of cellulose
compared to others
- Differences indicate that cellulose walls in plants and charophyceans evolved
independently of those in other algae
2. -Peroxisomes of both land plants and charophyceans contain enzymes that
minimize the loss of organic product
3. Structure of flagellated sperm (same between the two)
4. Formation of a phragmoplast (Same cell division)
- The synthesis of new cross walls during cell division involves the formation of a
phragmoplast
Mickle Notes
• Same pigments and chlorophyll's between the two
• Food storage= Startch (also in green algae)
• Green algae have the same number of cell divisions
Adaptations Enabling the move to land
• Green algae inhabit shallow waters
• Natural Selection favors individual algae that can survive when not in water
• In green algae, a layer of durable polymer called sporopollenin prevents exposed
zygotes from drying out
• Environmental changes- brighter sunlight, atmosphere had an abundance of CO2, soil was
rich in nutrients, fewer herbivores and pathogens
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Traits of Plants
• 5 key traits absent in green algae
a. apical meristems
b. Alternation of generations
c. Walled spores produced in sporangia
d. Multicellular gametangia
e. Multicellular, dependent embryos
A. Apical Meristems
Roots and shoots can elongate, increasing exposure to environmental resources
This growth in length is sustained throughout the plant’s life by the activity of :
Apical meristems- localized regions of cell division at the tips of shoots and roots
- Cells produced y apical meristems differentiate into various tissues, including a
surface epidermis's that protects the body and several types of internal tissues.
B. Alternation of Generations
Life cycles of land plants alternate between 2 different multicellular bodies
It does not occur in charophyceans. We can infer that it is a characteristic derived of land
plants- it was not present in the ancestor common to land plants and charophyceans
Alternation of generations is distinguished by the fact that there are both
multicellular haploid and diploid stages in their life cycle
The 2 multicellular forms that alternate in the cycles are: Gametophytes and
Sporophyte generations:
• Gametophyte cells are haploid, its named for its production by mitosis of haploid
gametes (egg and sperm) that fuse together forming diploid zygotes
...
Mitotic division of the zygote produces the multicellular:
• Sporophyte, thus those cells are diploid. Meiosis in a mature sporophyte produces
haploid spores- these cells can develop into a new organism without fusing with another
cell
• Gametes cannot develop directly into a multicellular organism, but are ment to fuse and
form zygotes
• When a spore undergoes mitosis, it makes a new multicellular gametophyte and so the
cycle continues from there.
C. Walled spores produced in the Sporangia
Sporopollenin makes walls of plant spores very tough and resistant to harsh
environments, this chemical adaptation makes it possible for spores to be dispersed
through dry air without harm.
A Sporophyte contains a sporangium, within that exist diploid cells called sporocytes
(spore mother cells) undergo meiosis and generate those haploid spores
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Charophyceans also produce spores, but these algae lack multicellular sporangia and their
flagellated, water dispersed spores lack sporopollenin
D.
Multicellular Gametangia
Production of gametes within gametangia is another feature of land plants and not
their algal ancestors. The female gametangia are called archegonia, each of which are a
vase shaped organ that produces a single egg retained within the base of the organ. Male
gametangia called antheridia produces and releases sperm into the environment. Each
egg is fertilized within an archegonium where the zygote develops into an embryo,
E. Multicellular, Dependent Embryos
Embryos develop from zygotes, and are retained within tissues of the female parent.
The parental tissues provide the embryo with nutrients, such as sugars and amino acids.
The embryo has specialized placental transfer cells- sometimes present in the adjacent
maternal tissue as well- which enhance the transfer of nutrients from parent to
embryo though the elaborate ingrowths of the wall surface (plasma membrane and cell
wall). This trait is so significant and derived from land plants that they are also known as
embryophytes.
• These traits were absent in the ancestor common to land plants and Green Algae
• Set land plants apart from their closest algal relatives
Other traits that relate to terrestrial plant life are:
1. Greater risks of desiccation (drying out)-- their surfaces have a cuticle, that consists of
polymers called polyesters and waxes. Acts as waterproofing, and protects against
microbial attack
2. Many land plants produce molecules called secondary compounds, Includes: alkaloids,
terpenes, tannins and phenolics such as flavonoids.
- various alkaloids, terpenes and tannins have a bitter taste, strong odor or toxic
effect that help defend against herbivores and parasites
- Flavonoids absorb harmful UV radiation and may act as signals in symbiotic
relationships with beneficial soil microbes
- Phenolics deter attack by pathogenic microbes
- Humans also benefit from secondary compounds, (ex. alkaloids that can fight
malaria)
Origin and diversification of plants
• Fossil spores dating up 475 mya
• Some spores are different from typical land plants
- spores of living plants dispersed as single grains, but fossil spores are fused
together in groups of 2 or 4
- Raises the possibility that the fossil spores were not produced by plants, but by
some extinct algal relative
• The oldest known fragments of plant tissue are 50 million years younger than the
puzzling spores
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Land Plants
Grouping
Based on the presence or absence of vascular tissue - cells joined into tubes that transport
water and nutrients throughout the plant body.
• Vascular plants - have a complex vascular tissue system.
• Non vascular plants: plants that do not have an extensive transport system (ex.
liverworts, hornworts, moss) ( some moss have simple vascular tissue)
• Non-Vascular plants are often informally called bryophytes (meaning moss-plant in
greek). Whether or not bryophytes are monophyletic, they share some derived traits
with vascular plants such as: multicellular embryos, and apical meristems (but they
lack true roots and leaves)
• Vascular plants form a clade, and consist of about 93% of all plant species. They can
be further characterized into 3 smaller clades. 2 of these clades are the
• Lycophytes (club moss and their relatives) ..and..
• Pterophytes(ferns and their relatives).
Each of these clades lack seeds, which is why collectively the two clades are often
informally called seedless vascular plants. The third clade are seed plants, the vast
majority of living plants species. A seed is an embryo packaged with a supply of nutrients
inside its coat. Those seed plants can then be divded into 2 groups, Gymnosperms and
angiosperms. Based on the absence or presence of enclosed chambers in which seeds
mature.
• Gymnosperms- are grouped together as “naked” seed plants. Their seeds are not
enclosed in chambers. Of these surviving species include conifers, probably form a
clade
• Angiosperms- are a huge clade consisting of all flowering plants. Angiosperm seeds
develop inside chambers called ovaries, which originate within flowers and mature
into fruits.
The life cycle of moss and other bryophytes are dominated by the
gametophyte stage
3 small phyla of herbaceous (non-woody) plants that represent bryophytes are:
a. Liverworts (phylum hepatophyta)
b. Hornworts (phylum anthocerophyta)
c. Moss (bryophyta)
A and B are named after their shape, wort means herb. Mosses are our most familiar
bryophyes, although some plants commonly called moss are not really moss at all. (this
includes irish moss (a red type of sea weed), reindeer moss (a lichen) and club moss
(seedless vascular plants)
..Don’t Confuse
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Bryophyta is the formal taxonomic name for the phylim, and only means mosses.
Bryophyte is used informally to refer to all nonvascular plants (a,b and c)
Note: it is not established if (a,b, and c form a clade)
Living bryophytes reflect some traits of the earliest plants, and have many unique
adaptations. The oldest known fossils of plant fragments as ex. have the same type of tissue
as liverworts.
Bryophyte Gametophytes
• In bryophytes (all 3) have larger and longer living gametophytes than sporophytes.
Sporophytes are present in only part of the cycle time.
• If spores are dispersed in a favorable habitat, (moist soil or tree bark) they may
germinate and grow into gametophytes.
• In moss spores, they characteristically produce a mass of green, branched, one cell thick
filaments known as protonema. A protonema has a large surface area that enhances
absorption of water and minerals.
- in favorable conditions, a protonema produces one or more “buds” each with an
apical meristem that generates a gamete-producing structure known as a
gametophore. Together, a protonema and gametophore make up the body of a
moss gametophyte.
Gametophytes have ground hugging few celled thick carpets, that can’t really support tall
plants.Bryophytes also don’t have vascular tissue, which is required for long distance
transport of water and nutrients, so it’s not that tall.
Rhizoids anchor gametophytes, they are long, tubular single cells (in liverworts and
hornworts) or filaments of cells (in moss). Unlike roots, rhizoids are not composed of
tissuies, lack specialized conducing cells and do not play a primary role in water and
mineral absorption.
Mature gametophytes produce gametes in gametangia covered by protective tissue.
A gametophyte may have multiple gametangia. Eggs are produced singly in archegonia,
where antheridia makes sperm. Some bryophyte gametophytes are bisexual, but in moss
the archegonia and antheridia are typically carried on separate F and M gametophytes.
Flagellated sperm typically swim through a film of water towards eggs, entering the
archegonia (attracted to chemicals). Eggs are not released but instead remain within the
base of the archegonia. layers of placental transfer cells help transport nutrient's to the
embryos as they develop into sporophytes.
Bryophyte Sporophytes
Sporophytes are usually green and photosynthetic when young and cannot live
independently. They are attached to their parental gametophytes and absorb nutrients.
(sugars, amino acids, minerals and water)
Of all the plant groups, bryophytes have the smallest and simplest sporophytes (gives truth
to the hypothesis more complex sporophytes evolved later in vascular plants).
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A sporophyte consists of a foot, seta, and a sporangium. Embedded in the archegonium,
the foot absorbs the nutrients from the gametophyte. The seta, is the stalk, and conducts
the materials to the sporangium, which is also called a capsule, and from there produces
spores by meiosis.
The seta is elongated in moss for better spore dispersal. An immature capsule has a
protective cap of gametophyte tissue called a calyptra, which is shed when the capsule is
mature. The upper part of the capsule sometimes contains a ring of toothlike structures
known as the peristome. Its specialized for gradual spore discharge, using wind that can
carry spores.
Hornwort and moss sporophytes are larger and more complex than those of
liverworts. Both Horn w. and moss have specialized pores called stomata, which are also
found in all vascular plants.
This suggests three possible hypothesis for their evolution.
Liverworts (phylum hepatophyta)
• liver-shaped gametophytes
• described as “thalloid” because of the flattened shape of their gametophytes
• Some are called “leafy” because their gametophytes have a stemlike structure with many
leaflike appendages.
• Found in tropical regions, they are much more common than thalloid species
Hornworts-(phylum anthocerophyta)
Shape refers to the sporophyte, which resembles a small grass blade
The sporangium extends along its length and splits open to release mature spores, starting
at the tip of the horn.
Gametophytes grow horizontally and often have multiple sporophytes attached
Moss (phylum bryophyta)
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As opposed to the worts, moss grows vertically than horizontally
gametophytes are the structures that primarily comprise a carpet of moss
their “leaves” are usually only one cell thick,
Moss sporophytes are typically elongated and visible to the naked eye
though green and photosynthetic when young, they turn tan or brownish when ready to
release spores
Ecological and Economic Importance of Moss
• Some moss can inhabit such extreme environmental conditions such as mountaintops,
tundra and deserts
• Many exist in very cold or dry habitats/c they can survive the loss of most of their body
water, then rehydrate when moisture is available
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• Phenolic compounds in moss cell walls absorb levels of radiation present in deserts or
at high altitudes and latitudes
Peat moss,
• or Sphagnum, is a wetland moss and produces partially decayed organic material known
as peat
• Because of the phenolic compounds embedded in its cell walls, sphagnum does not decay
readily. It also secretes compounds that may reduce bacterial activity.
• Low temp. and nutrient levels in peat bogs also inhibit decay. As a result, peat bogs can
preserve mummified corpses for thousands of years
In a larger scale...
• 400 billion tons of organic carbon are stored in peat. These carbon reservoirs help
stabilize global atmospheric CO2 concentrations
• Peat is also a fuel source in europe and asia, it can also serve as soil conditioner and is
used for packing plant roots during shipment